Significant work is ongoing to reveal how different cell types in the brain contribute to behavior and pathology, and how they change in plasticity and disease, empowered by new genetic, optical, and physiological tools. However, the functional activity and dysregulation of neuronal circuits relies critically on the in situ molecular composition of neuronal synapses. Although it is clear that the properties of a given synapse are determined by, amongst other things, the specific types of cells that are thus connected, far less is known about the diversity of synapse types in the brain than cell types, perhaps because this is an intrinsically proteomic problem: a given neuron might make many different kinds of synapse with different targets, and thus transcriptomics (which is prevailing as a method for cell type analysis) may not suffice for synapse typing. High- throughput in situ proteomic tools are needed to characterize synapse molecular composition at the single-cell level in the context of whole brains or brain regions, and thus to connect the currently distant topics of neuronal activity and genetic aberrations associated with disease pathology. Here, we propose a high-risk, high-payoff, and as far as we know entirely novel agenda: to develop tools capable of resolving the molecular proteomic composition of synapse types, testing them in cultured neurons and intact brain tissues. To achieve this transformative goal of establishing a broadly useful tool for in situ synapse proteomics, we will build on our recent breakthrough in developing the DNA-based highly multiplexed, quantitative super-resolution imaging method DNA- PAINT (Points Accumulation for Imaging in Nanoscale Topography). DNA-PAINT exploits the transient binding of short fluorescently labeled DNA-probes for simple and easy-to-implement quantitative, highly multiplexed, super-resolution imaging with sub-10 nm resolution. In this application, we plan to develop and apply DNA-PAINT to enable quantitative, ultra-multiplexed, in situ characterization of neuronal synapse proteins for understanding synaptic types and studying cell type-specific synaptic functions. The outcome of our work will be a broadly useful in situ proteomic tool for quantification of neuronal synapse composition that can be used by diverse neurobiology laboratories to study single cell-level synapse properties in fixed tissues from whole brains or cell culture assays.